[Twisted-web] Fixing a failing test

[Valentino Volonghi]

Fixed a failing test added by mg with r296. 

It was due to not detecting properly if l was already root.

Run test_url.py and all 10 tests passed.

--- url.py      (revision 296)
+++ url.py      (working copy)
@@ -82,6 +82,10 @@
             l.append(path)
         return self._pathMod(l, self.queryList(0))
  
+    def isRoot(self, pathlist):
+        if len(pathlist) > 2: return False
+        return True
+
     def parent(self):
         """Construct a url which is a parent of this url;
         This is logically equivalent to '..' of the current url.
@@ -99,7 +103,8 @@
             # we are a file, such as http://example.com/foo/bar our
             # parent directory is http://example.com/
             l.pop()
-            l[-1] = ''
+            if self.isRoot(l): l.append('')
+            else: l[-1] = ''
         return self._pathMod(l, self.queryList(0))
  
     def here(self):